Implement enableFastMobileScrolling web api.

The effort needs to be done in 3 steps:
1. Create the runtime flag interface in blink;
2. Enable the flag for Android from chromium even it does not do anything in blink;
3. Replace compile time guard with runtime flag in blink and remove ENABLE_FAST_MOBILE_SCROLLING in gyp file.

This is the 1st set of changes.

BUG=381391

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175621

Review URL: https://codereview.chromium.org/321473002

git-svn-id: svn://svn.chromium.org/blink/trunk@175679 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -56,6 +56,7 @@ EncryptedMediaAnyVersion status=stable
 ExperimentalCanvasFeatures status=test
 ExperimentalContentSecurityPolicyFeatures status=experimental
 ExperimentalWebSocket status=stable
+FastMobileScrolling
 FileAPIBlobClose status=experimental
 FileConstructor status=experimental
 FileSystem status=stable

third_party/WebKit/Source/web/WebRuntimeFeatures.cpp

@@ -105,6 +105,11 @@ void WebRuntimeFeatures::enableExperimentalCanvasFeatures(bool enable)
     RuntimeEnabledFeatures::setExperimentalCanvasFeaturesEnabled(enable);
 }
 
+void WebRuntimeFeatures::enableFastMobileScrolling(bool enable)
+{
+    RuntimeEnabledFeatures::setFastMobileScrollingEnabled(enable);
+}
+
 void WebRuntimeFeatures::enableFastTextAutosizing(bool enable)
 {
     RuntimeEnabledFeatures::setFastTextAutosizingEnabled(enable);

third_party/WebKit/public/web/WebRuntimeFeatures.h

@@ -58,6 +58,8 @@ public:
 
     BLINK_EXPORT static void enableExperimentalCanvasFeatures(bool);
 
+    BLINK_EXPORT static void enableFastMobileScrolling(bool);
+
     BLINK_EXPORT static void enableFastTextAutosizing(bool);
 
     BLINK_EXPORT static void enableFileSystem(bool);